Concentrating Solar Power (CSP) is a renewable energy technology that utilizes solar thermal power generation to convert solar thermal energy to electricity. The solar thermal power generation process generally involves converting the solar energy into thermal energy, then converting the thermal energy into mechanical energy, and then converting the mechanical energy into electricity. CSP power plants achieve high-efficiency solar thermal power generation by way of utilizing a solar concentrator (e.g., a parabolic trough, a solar dish, or an array of mirrors) to direct concentrated sunlight onto a solar receiver (i.e., a structure comprising a sunlight absorbing material), thereby converting the solar energy into thermal energy by way of heating the solar receiver to a high operating temperature. The thermal energy is then converted into mechanical energy by way of generating high temperature steam (e.g., by forming the light absorbing material of the solar receiver into a pipe-like structure, and passing water/steam through the pipe's conduit). The high temperature steam is then directed to a turbine, whereby the mechanical energy is converted to electricity by a conventional generator driven by the turbine. Excess thermal energy is often collected in molten salts and stored in large insulated tanks, allowing operation of the steam turbine during the night or on cloudy days. CSP power plants are therefore similar to most conventional power plants that utilize heat from a fuel source to drive a stream turbine, but instead of generating heat, e.g., from the combustion of fossil fuels or another non-renewable fuel source, solar thermal power plants produce steam using sunlight.
The evolution of CSP technology is mainly driven by the goal of achieving higher solar receiver operating temperatures, with central receiver CSP plants currently recognized as being superior in this regard over parabolic trough and solar dish CSP plants. Solar receiver operating temperatures above about 500° C., and more preferably above 600° C., are optimal for achieving high power cycle efficiency, reduce material costs for thermal storage, and lower the overall cost of electricity generated by a CSP plant. Parabolic trough CSP plants utilize elongated half-cylinder-shaped parabolic reflectors to concentrate sunlight onto line-type (pipe-like) solar receivers positioned along a focal line defined by each parabolic reflector. Because the parabolic shape focuses sunlight in only one plane, the concentration ratio of parabolic reflectors is limited to 30 to 100 times its normal intensity, whereby parabolic trough CSP plants achieve relatively low maximum operating temperatures (i.e., approximately 400 to 550° C.). Solar dish CSP plants utilize a dish (bowl) shaped reflector that concentrates incident sunlight on a point-type solar receiver located at a focal point of the dish, thereby achieving higher concentration ratios than can be achieved using a parabolic trough reflector, whereby solar dish CSP plants achieving relatively high maximum operating temperatures (i.e., 600-750° C.). However, the solar dish reflector must be constantly repositioned throughout the day to maintain focus of the concentrated sunlight on the solar receiver, so practical considerations associated with supporting and moving solar dishes limit their practical size and corresponding weight, thereby limiting the peak power generation capacity of solar dish CSP plants. Central receiver CSP plants utilize an array of heliostats disposed on the ground next to a tower supporting a solar receiver, where each heliostat includes a mirror and an associated positioning mechanism that repositions the mirror throughout the day to reflect incident sunlight onto the solar receiver. Similar to solar dish CSP plants, the peak power generation capacity of central receiver CSP plants is determined by the amount of sunlight reflected onto the tower-based receiver, which in both cases corresponds to the total reflective area of the mirrors. However, unlike solar dish reflectors whose practical size and weight are limiting factors, the total reflective area of a central receiver CSP system is expandable by installing additional heliostats next to the receiver tower; that is, because each heliostat is disposed on the ground and includes its own mirror positioning mechanism, expanding the reflective area of a central receiver CSP plant merely requires installing an aligning additional heliostats. Existing central receiver CSP plants collect concentrated sunlight reflected from hundreds or thousands of heliostats to achieve concentration ratios as high as 1,500 times the sun's normal intensity, and maximum operating temperatures well above 750° C. Central receiver CSP plants are therefore capable of achieving substantially higher solar receiver operating temperatures than parabolic trough and solar dish CSP plants, and hence are currently the preferred CSP plant technology for large volume electricity generation.
Although advances in CSP technology facilitate the generation of optimal solar receiver operating temperatures (i.e., above about 600° C.), a current limiting factor to further advances in CSP technology is the inability of presently available sunlight absorbing materials to absorb solar energy with high efficiency at these temperatures. As mentioned above, a solar receiver is a structure consisting of a sunlight absorbing material that is configured to transfer absorbed heat to a heat transfer fluid (e.g., molten salt or water/steam). A solar receiver's efficiency is defined by the amount of received solar energy that is absorbed/transferred to the heat transfer fluid versus the amount of received solar energy that is lost due to convective heat losses (e.g., due to wind and buoyancy effects) and radiative heat losses (i.e., as materials get hot, heat energy is radiated away at infrared wavelengths to the surrounding environment). At a 600° C. operating temperature, the primary cause of heat loss from CSP plant's solar receiver is thermal radiation losses, and the rate of radiative heat loss increases as operating temperatures increase above 600° C. Therefore, although central receiver CSP plants are capable of achieving operating temperatures well above 600° C., radiative heat loss from solar receivers at higher operating temperatures limits the overall efficiency of a central receiver CSP plant operating at higher temperatures.
Current efforts to reduce radiative heat loss from solar receivers operating above 600° C. are focused on developing spectrally-selective coatings that are painted or otherwise applied to the solar receiver's “core” sunlight absorbing material. These coatings are formulated to maximize solar absorptance in the visible and near-infrared wavelengths (i.e., approximately 400 to 2500 nm) while minimizing thermal emittance in the infrared wavelengths (i.e., approximately 1 to 20 microns). Because these spectra overlap, especially at higher temperatures, development of selective coatings is challenging. Additionally, these selective absorber coatings need to be durable at high temperatures in exposed environments to avoid degradation. Currently, Pyromark® Series 2500 Flat Black (LA-CO Industries Inc., Elk Grove Village, Ill., USA), which is a high temperature resistant silicone coating exhibiting high solar absorptance (>95%), has been recognized by Sandia National Laboratories as a standard for use on solar receivers in central receiver CSP plants (see Levelized Cost of Coating (LCOC) for Selective Absorber Materials, Clifford K. Ho and James E. Pacheco, SANDIA REPORT SAND2013-8327, Printed September 2013). However, Pyromark® Series 2500 also exhibits a thermal emittance of 0.87 and suffers from large thermal losses during high temperature operation, and also exhibited significant degradation at higher temperatures (>700° C.) when operated in air, causing a decline in performance and potentially added operating costs for CSP plants. Further, the coating-based approach suffers from inherent delamination issues due to the unavoidable differences between the coating material and the underlying sunlight absorbing “core” material forming the solar receiver, and the coating material re-emits in the IR, which lowers thermal efficiency.
What is needed is a solar receiver that exhibits high absorptance (i.e., >95%) and low thermal emittance (i.e., <10%) of solar energy at operating temperatures above 600° C., and also avoids the problems associated with conventional coatings-based approaches.